Universität zu Köln

Perez-Beaupuits, J. P., Guesten, R., Spaans, M., Ossenkopf, V., Menten, K. M., Requena-Torres, M. A., Wiesemeyer, H., Stutzki, J., Guevara, C. and Simon, R. (2015). Disentangling the excitation conditions of the dense gas in M17 SW. Astron. Astrophys., 583. LES ULIS CEDEX A: EDP SCIENCES S A. ISSN 1432-0746

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Abstract

Context. Stars are formed in dense molecular clouds. These dense clouds experience radiative feedback from UV photons, and X-ray from stars, embedded prestellar cores, young stellar objects (YSOs), and ultra compact H II regions. This radiative feedback affects the chemistry and thermodynamics of the gas. Aims. We aim to probe the chemical and energetic conditions created by radiative feedback through observations of multiple CO, HCN, and HCO+ transitions. We measure the spatial distribution and excitation of the dense gas (n(H-2) > 10(4) cm(-3)) in the core region of M17 SW and aim to investigate the influence of UV radiation fields. Methods. We used the dual band receiver GREAT on board the SOFIA airborne telescope to obtain a 5.'7 x 3.'7 map of the J = 16 -> 15, J = 12 -> 11, and J = 11 -> 10 transitions of (CO)-C-12 in M17 SW. We compare these maps with corresponding APEX and IRAM 30m telescope data for low-and mid-J CO, HCN, and HCO+ emission lines, including maps of the HCN J = 8 -> 7 and HCO+ J = 9 -> 8 transitions. The excitation conditions of (CO)-C-12, HCO+, and HCN are estimated with a two-phase non-LTE radiative transfer model of the line spectral energy distributions (LSEDs) at four selected positions. The energy balance at these positions is also studied. Results. We obtained extensive LSEDs for the CO, HCN, and HCO+ molecules toward M17 SW. These LSEDs can be fit simultaneously using the same density and temperature in the two-phase models and to the spectra of all three molecules over a similar to 12 square arc minute size region of M17 SW. Temperatures of up to 240 K are found toward the position of the peak emission of the (CO)-C-12 J = 16 -> 15 line. High densities of 10(6) cm(-3) were found at the position of the peak HCN J = 8 -> 7 emission. Conclusions. We found HCO+/HCN line ratios larger than unity, which can be explained by a lower excitation temperature of the higher-J HCN lines. The LSED shape, particularly the high-J tail of the CO lines observed with SOFIA/GREAT, is distinctive for the underlying excitation conditions. The cloudlets associated with the cold component of the models are magnetically subcritical and supervirial at most of the selected positions. The warm cloudlets instead are all supercritical and also supervirial. The critical magnetic field criterion implies that the cold cloudlets at two positions are partially controlled by processes that create and dissipate internal motions. Supersonic but sub-Alfvenic velocities in the cold component at most selected positions indicate that internal motions are likely magnetohydrodynamic (MHD) waves. Magnetic pressure dominates thermal pressure in both gas components at all selected positions, assuming random orientation of the magnetic field. The magnetic pressure of a constant magnetic field throughout all the gas phases can support the total internal pressure of the cold components, but cannot support the internal pressure of the warm components. If the magnetic field scales as B proportional to n(2/3), then the evolution of the cold cloudlets at two selected positions, and the warm cloudlets at all selected positions, is determined by ambipolar diffusion.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code Perez-Beaupuits, J. P. UNSPECIFIED UNSPECIFIED UNSPECIFIED Guesten, R. UNSPECIFIED UNSPECIFIED UNSPECIFIED Spaans, M. UNSPECIFIED UNSPECIFIED UNSPECIFIED Ossenkopf, V. UNSPECIFIED UNSPECIFIED UNSPECIFIED Menten, K. M. UNSPECIFIED UNSPECIFIED UNSPECIFIED Requena-Torres, M. A. UNSPECIFIED UNSPECIFIED UNSPECIFIED Wiesemeyer, H. UNSPECIFIED UNSPECIFIED UNSPECIFIED Stutzki, J. UNSPECIFIED UNSPECIFIED UNSPECIFIED Guevara, C. UNSPECIFIED UNSPECIFIED UNSPECIFIED Simon, R. UNSPECIFIED UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-387877
DOI:	10.1051/0004-6361/201526441
Journal or Publication Title:	Astron. Astrophys.
Volume:	583
Date:	2015
Publisher:	EDP SCIENCES S A
Place of Publication:	LES ULIS CEDEX A
ISSN:	1432-0746
Language:	English
Faculty:	Unspecified
Divisions:	Unspecified
Subjects:	no entry
Uncontrolled Keywords:	Keywords Language ABSORPTION-LINE SURVEY; HIGH-RESOLUTION; MOLECULAR GAS; ZEEMAN OBSERVATIONS; MAGNETIC-FIELDS; STAR-FORMATION; ATOMIC CARBON; PHOTODISSOCIATION REGION; CHEMICAL DIFFERENTIATION; ISOTOPE FRACTIONATION Multiple languages Astronomy & Astrophysics Multiple languages
URI:	http://kups.ub.uni-koeln.de/id/eprint/38787